Add roundrobin middleware

This middleware allows you to round robin a/aaaa records in a reply and
maybe more in the future (i.e.) sort a packet?

middleware/loadbalance/handler.go

@@ -0,0 +1,19 @@
+// Package loadbalance is middleware for rewriting responses to do "load balancing"
+package loadbalance
+
+import (
+	"github.com/miekg/coredns/middleware"
+	"github.com/miekg/dns"
+	"golang.org/x/net/context"
+)
+
+// RoundRobin is middleware to rewrite responses for "load balancing".
+type RoundRobin struct {
+	Next middleware.Handler
+}
+
+// ServeHTTP implements the middleware.Handler interface.
+func (rr RoundRobin) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg) (int, error) {
+	wrr := NewRoundRobinResponseWriter(w)
+	return rr.Next.ServeDNS(ctx, wrr, r)
+}